Production of cyclohexane



Oct. 9, 1956 R. B. BENNETT ET AL 2,766,303

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INVENTDR.

United States Patent 9 2,766,303 rnoniicrroN or CYCLOHEXANE Rufus B. Bennett and Reuben F. Pfennig, Baytown, Tern, assignors, by mesne assignments, to Esso Research and Engineering (Jornpany, Elizabeth, N. J., a corporation of Delaware Application April 4, 1%52, erial No. 280,604 4 Claims. (Cl. 260-666) not more than 88% by volume of cyclohexane in admixture with ncri-naphthenic hydrocarbons to a temperature in the range between -10 and 130 F. to form a slurry of cyclohexane crystals in a mother liquor. The slurry of cyclohexane crystals is then treated to recover a crystalline cake of cyclohexane therefrom.

The cyclohexane crystals may be recovered from the slurry by filtering or centrifuging the slurry under conditions to form crystalline cakes having a cyclohexane content of at least 95%. Actually it has been possible to recover crystalline cakes of cyclo-hexane of 99% purity.

The feed stock is suitably a hydrocarbon fraction containing cyclohexane, boiling in the range from 170 to 180 F. and containing at least 70% by volume of cyclohexane. The cyclohei'iane-containing fraction should contain no more than 88% by volume of cyclohexane. Actually the upper limit of purity is dictated by the practical limits of commercially available distillation units. It is possible to recover by fractional distillation cyclohexane having a purity in the range from 85% to 88% by volume.

The cyclohexane containing fraction having the purity indicated is chilled to a temperature in the range from 10 to 130 F. to form a slurry of cyclohexane crystals in a mother liquor. The cycloheXane-containing fraction containing a lower amount of cyclohexane in the range given Willbe chilled to a lower temperature in the range indicated while the higher purity cycloheXane-containing fractions may be chilled to higher temperatures in the range given. For example, a cyclohcXane-containing fraction containing 85% cyclohexane may be chilled to form a slurry at a temperature in the range between 10 and -60 F. and still allow recovery of cyclohexane of 95% or better purity.

The impurities in cycloheXane-containing fractions encountered in the modern petroleum refineries equipped with precise fractional distillation units are principally dimethylpentanes and a trace of benzene. Ordinarily, the amount of benzene in such fractions will be of the order of 5% to by volume and are handled Without difficulty. However, when the amount of benzene in the cyclohexane-containing fractions is greater than 10% and ranges as high as by volume, it will be desirable to crystallize at the lower temperatures in the range given.

Dimethylpentanes have freezing points below -100 F. while benzene freezes at 42 F. Cyclohexane freezes at 44 F. It has been found that the crystallization point of the solution containing 75% cyclohexane in isoheptanes is 38 F. whereas the crystallization point of a solution of 50% cycloheXane in isoheptanes was not reached at a temperature as low as l00 F. Thus in accordance with the present invention it is essential that the cyclolreitanecontaining fraction charged to the chilling stage of the present invention contain at least 70% cyclohexane to obtain the desired purity.

The invention will be further illustrated by reference to the drawing in which Figure 1 is a flow diagram showing one mode of practicing the invention, and

Figure 2 is a modification of Figure 1.

Referring now to the drawing, numeral 11 designates a line through which a cyclohexane-containing fraction boiling in the range from to F. and containing, for example, 13% cyclohexane is introduced into the system. Line ll connects into a fractional distillation zone 12- which may be a plurality of fractional distillation towers. For purposes of this description, however, it will be assumed that fractional distillation zone 12 is a single fractional distillation tower provided with suitable internal baffling equipment to allow close fractionation to obtain a fraction containing at least 70% and no more than 88% by volume of cyclohexane. Such fractional distillation towers are available in the industry and are known as super-fractionation towers. in any event, fractional distillation zone 12 is provided with a heating means illustrated by coil 13 to allow adjustment of temperature and pressure and to allow the obtaining of a fraction containing the quantity of cyclohexane indicated as a side stream by line 14. A bottoms fraction is dis charged from 12 by line 10 for use as may be desired. The side stream fraction is withdrawn from zone 12 by line 14 and is introduced thereby into a suitable chilling means illustrated by scraped surface chiller 15 which is shown with an internal scraper to which removes the crystals from the interior surface of the chiller as they are formed and conducts them to the middle thereof and thus outwardly from the chiller. Although not shown, scraped surface chiller 15 is equipped with suitable chilling means such as a jacket and the like. It is understood that other chilling devices may be used such as one of the Gslo crystallizer type. it is also understood that internal refrigeration may be used to allow obtaining of the requisite temperatures. If desired, the cyclohexanecontaining fraction in line 14 may be internally refrigerated by adding a suitable refrigerant thereto, such as carbon dioxide, propane and the like, and then evapcrating the internal refrigerant. The chilling operation is conducted to form a slurry having a temperature in the range from 10 to -100 F. and the chiller is operated at such a temperature to form the slurry. Crystals of cyclohexane are formed in a mother liquor of the nonnaphthenic hydrocarbons and the slurry containing the cyclohexane crystals is withdrawn from the chiller-15 by line 17. if desired, the slurry may be discharged by branch line 1% controlled by valve 19 to a suitable holding tank 2% which is provided with a jacket 21 through which the cooling fluid may be circulated by lines 22. and 23. in holding tank 2 3 the crystals are allowed to grow and assume a size and shape which will allow ready separation from the mother liquor.

It may be desirable and'sometimes preferable to by-pass the holding tank 2th and under these conditions valve 19 in line 18 will be closed and valve 24 in line 17 will be opened allowing the chilled slurry to by-pass holding tank 26.

Whether or not holding tank 20 is by-passed, the slurry is discharged by line 25, into which line 17 connects, into a separating means illustrated by a centrifuge 26. Centrifuge 26 is provided witha jacket 27 through which a cooling fiuid may be circulated by lines 28 and 29. Centrifuge 26 allows a separation to be made between the cyclohexane crystals and the mother liquor, the mother liquor being withdrawn by line 30 and recycled thereby to line 1-1 for recovery of cyclohexane which will be contained therein. The cyclohexane will be formed into a cake in centrifuge 26 which is of the conventional type and may be recovered therefrom by a suitable doctor or knife blade or by melting and the like. The cyclohexane is Withdrawn from centrifuge 2.6 by line 31 and is of 95% or better purity.

Other separation means may be used for centrifuge 26. In Fig. 2 centrifuge 26 is replaced by a filter or plurality of filters 40. Line connects to filter 40 to allow the slurry to be introduced therein and the filter, which may be of the leaf or rotary type, causes separation of the cyclohexane crystals from the mother liquor, the mother liquor being withdrawn from the filter 40 by line 30 while the crystalline cake of cyclohexane is discharged as such, or melted, by line 31 for recovery thereof.

The mother liquor may contain an appreciable concentration of dimethyl pentanes which originally was present in the feed. This mother liquor, containing only a relatively small amount of the original cyclohexane charged, may be withdrawn from the system by line 32, controlled by valve 33, in which case valve 34 remains closed. In a preferred method of operation, however, valve 34 is opened and at least a portion of the mother liquor is recycled to fractional distillation Zone 12. In order to avoid excessive build up of dimethyl pentanes in the system, a fraction including an azeotrope containing about 37.5 mol percent cyclohexane and 62.5 mol percent dimethyl pentanes may be discarded by line 35 as overhead from the distillation zone.

The foregoing description taken with the drawing provides a simple, readily adaptable process whereby high purity cyclohexane may be recovered from fractions containing it. Cyclohexane is commercially important since it is used in the manufacture of nylon. It is also suitable for use in manufacturing chemicals and chemical intermediates.

The invention will be further illustrated by the following examples:

A cyclohexane concentrate containing 85% by weight (about 83% by volume) of cyclohexane was chilled in a scraped surface chiller to form a slurry and subsequently centrifuged to produce high purity cyclohexane. The centrifuging operation was conducted at the temperature of the slurry employing a centrifuge operating at a speed sufficient to exert a force of 750 times gravity. In a first run the cyclohexane concentrate was chilled for 3% minutes to form a slurry at -20 F. Itwas then centrifuged for 2 minutes under the conditions specified to form a cake having a thickness of 1". The cake was recovered and it was found that the yield based on the cyclohexane concentrate was 30 weight percent. The purity of the cyclohexane recovered from the cake was 96%, so that the yield expressed as 100% cyclohexane was 28.8 weight percent.

In a second run the cyclohexane concentrate of 85% by weight (about 83% by volume) purity was chilled for 8 minutes to form a slurry at a temperature of F. The chilled slurry was then centrifuged for 2 minutes at a temperature of 40 F. to form a cake of cyclohexane having a thickness of l. The cake of cyclohexane was recovered at a yield of 33% having a purity of 95.5% cyclohexane. The yield of cyclohexane on a 100% basis was 31.5%. The foregoing runs show that crystallization and centrifuging of a fraction having 85% concentration of cyclohexane is effective in producing 95 to V to the feed fractionation step to remove dimethyl pentanes as azeotropes with cyclohexane.

Higher purity cyclohexane may be obtainable by melting the crystals, recrystallizing and recentrifuging.

In order to illustrate the invention further a cyclohexane concentrate also containing by weight cyclohexane was chilled to form slurries at temperatures ranging from -20 to F. and then subjected to filtration. The results of these runs are presented in the following table:

Table Cyclo- Filtration Cake hexane Slurry Temperature, F. Time, Thickness, Content of Minutes Inches Cake, M01

Percent M 96 600 36 99 900 99 420 it 98 110 -'%e 96 110 V6 99 360 /8 96 360 94 5 The data in the foregoing table indicate recovery of cyclohexane having a purity from 95% to 99%.

It is contemplated in the practice of the present invention that we may employ wash liquids, such as nonnaphthenic hydrocarbons having a boiling point substantially different from that of the cyclohexane. Examples of such wash liquids which we may use are liquefied propane, butane, pentane, ethyl alcohol, methyl alcohol and the like, as well as the higher members of the homologous series of the paraffins whose boiling points differ substantially from the boiling point of cyclohexane. Aromatic hydrocarbons meeting the requirements set out before may also be used as a Wash liquid. Paraflin hydrocarbons and mixtures thereof boiling above F. may be used.

It may also be desirable to hold the slurry in the holding tank such as illustrated in the description taken with the drawing for a time ranging from /3 minute up to about 30 minutes to allow formation of crystals of the desired size and shape conducive for separation of cyclohexane crystals from the mother liquor.

It is also contemplated that the cake formed in either the centrifuge or the filter may be subjected to a sweating operation similar to that used in the wax crystallization art. In short, the cake may be externally or internally heated to cause liquefaction of a portion of the cake which will allow recovery of the purified cake. External heating may be from a source of external heat, such as hot air or other fluid and internal heat may be provided by heating elements embedded in the filter surface or in the centrifuge.

The nature and objects of the present invention having been completely described and illustrated, What we wish to claim as new and useful and to secure by Letters Patent is:

1. A method for recovering cyclohexane having a purity of at least 95 which comprises distilling a hydrocarbon fraction boiling in the range between and F. and consisting of cyclohexane in admixture with non-naphthenic hydrocarbons including dimethyl pentanes to form a distillate azeotropic mixture of said cyclohexane and said dimethyl pentanes naturally present in said hydrocarbon fraction and a distillate second fraction containing at least 70% and not more than 88% by volume of cyclohexane, discarding said azeotropic mixture, chilling said second fraction to a temperature in the range between 10 and 100 F., forming a slurry of cyclohexane crystals in a mother liquor from said chilled second fraction, separating cyclohexane crystals of a purity of at least 95 from said mother liquor, and admixing at least a portion of said mother liquor with said hydrocarbon fraction.

2. A method for recovering cyclohexane having a pnrity of at least 95% which comprises distilling a hydrocarbon fraction boiling in the range between 170 and 180 F. and consisting of cyclohexane in admixture with nonnaphthenic hydrocarbons including dimethyl pentanes to form a distillate azeotropic mixture of said cyclohexane and said dimethyl pentanes naturally present in said hydrocarbon fraction and a distillate second fraction containing at least 70% and not more than 88% by volume of cyclohexane, discarding said azeotropic mixture, chilling said second fraction to a temperature in the range between and 100 F., forming a slurry of cyclohexane crystals in a mother liquor from said chilled second fraction, filtering said slurry to form a crystalline cake of cyclohexane of at least 95% purity, separately recovering said cake and said mother liquor, and admixing at least a portion of said mother liquor with said hydrocarbon fraction.

3. A method for recovering cyclohexane having a purity of at least 95 which comprises distilling a hydrocarbon fraction boiling in the range between 170 and 180 F. and consisting of cyclohexane in admixture with non-naphthenic hydrocarbons including dimethyl pentanes to form a distillate azeotropic mixture of said cyclohexane and said dimethyl pentanes naturally present in said hydrocarbon fraction and a distillate second fraction containing at least 70% and not more than 88% by volume of cyclohexane, discarding said azeotropic mixture, chilling said second fraction to a temperature in the range between -10 and -100 F., forming a slurry of cyclohexane crystals in a mother liquor from said chilled second fraction, centrifuging said slurry to form a crystalline cake of cyclohexane of at least purity, separately recovering said cake and said mother liquor, and admixing at least a portion of said mother liquor with said hydrocarbon fraction.

4. A method for recovering cyclohexane having a purity of at least 95 which comprises distilling a hydrocarbon fraction boiling in the range between and F. and consisting of cyclohexane in admixture with non-naphthenic hydrocarbons including dimethyl pentanes to form a distillate azeotropic mixture of said cyclohexane and said dimethyl pentanes naturally present in said hydrocarbon fraction and a distillate second fraction containing at least 70% and not more than 88% by volume of cyclohexane, discarding said azeotropic mixture, chilling said second fraction to a temperature in the range between -10 and 130 F., forming a slurry of cyclohexane crystals in a mother liquor from said chilled sec ond fraction, and separating cyclohexane crystals of a purity of at least 95 from said mother liquor.

References Cited in the file of this patent UNITED STATES PATENTS 2,373,673 Fuller et al. Apr. 17, 1945 2,435,792 McArdle et al. Feb. 10, 1948 2,540,977 Arnold Feb. 6, 1951 2,679,472 Tooke May 25, 1954 2,703,818 Cines Mar. 8, 1955 

1. A METHOD FOR RECOVERING CYCLOHEXANE HAVING A PURITY OF AT LEAST 95% WHICH COMPRISES DISTILLING A HYDROCARBON FRACTION BOILING IN THE RANGE BETWEEN 170* AND 180* F. AND CONSISTING OF CYCLOHEXANE IN ADMIXTURE WITH NON-NAPHTHENIC HYDROCARBONS INCLUDING DIMETHYL PENTANES TO FORM A DISTILLATE AZEOTROPIC MIXTURE OF SAID CYCLOHEXANE AND SAID DIMETHYL PENTANES NATUALLY PRESENT IN SAID HYDROCARBON FRACTION AND A DISTILALTE SECOND FRACTION CONTAINING AT LEAST 70% AND NOT MORE THAN 88% BY VOLUME OF CYCLOHEXANE, DISCARDING SAID AZEOTROPIC MIXTURE, CHILLING SAID SECOND FRACTION TO A TEMPERATURE IN THE RANGE BETWEEN -10* AND -100* F., FORMING A SLURRY OF CYCLOHEXANE CRYSTALS IN A MOTHER LIQUOR FROM SAID CHILLED SECOND FRACTION, SEPARATING CYCLOHEXANE CRYSTALS OF A PURITY OF AT LEAST 95% FROM SAID MOTHER LIQUOR, AND ADMIXING AT LEAST A PORTION OF SAID MOTHER LIQUOR WITH SAID HYDROCARBON FRACTION. 